1. Field of the Invention
This invention concerns a process for time-sequenced multiplexing of data on a transmission medium or carrier, more specifically for telephone messages coded by the pulse code modulation (PCM) technique. The invention also concerns multiplexing and demultiplexing devices used to perform this process.
Multiplexing is a method that allows a certain number of information channels or tracks to share a single physical medium or carrier, such as a cable or microwave link. This makes transmission much cheaper than when a separate medium is needed for every channel.
Such multiplexing can be achieved by distribution frequency ranges among the different channels, within the transmission band of the medium. This was the first method used with analog systems. A more recent solution consists of dividing the various channels by time sequence: this technique is used in telephony, in combination with PCM.
Pulse code modulation of a telephone signal involves sampling the analog signal for transmission, then quantizing the pulses resulting from such sampling, and finally coding them digitally; the digitized samples are then transmitted, and decoded at the receiver. When N channels are to be multiplexed on one carrier, sampling and coding are carried out successively for each channel and the resulting N digital samples, together with various auxiliary signals (synchronization, channel signalling, etc.), form what is usually described as a frame. In addition, it is often necessary to define a multiframe, comprising a series of M frames, mainly to simplify transmission of channel signalling data.
Such techniques have to some extent been internationally standardized, particularly as regards time multiplexing of 30 channels on a digital carrier with a capacity of 2,048 Kbits/sec (CCITT Recommendation F 732). For certain purposes, however, multiplexing of a smaller number of channels is preferable: the right compromise has to be found among system capacity, distance to be covered, and local needs. Multiplexing of 10 channels often constitutes such an optimum. The problem then arises of defining a frame and multiframe structure for a system with 10 multiplexed channels that will be compatible with the existing standard 30-channel system. It would be helpful to be able to combine three 10-channel systems in order to connect them to a 30-channel system, and conversely, break up a 30-channel system into 10-channel systems, without the resulting interfaces involving too much complexity or delay.
2. Description of the Prior Art
A number of solutions exist in the prior art. One such solution, for example, is described by Hans Jorg Frizlen and Walter Wild (in Ericsson Review No. 2, 1979). However, their performances are limited, in particular by the often serious delays affecting signals from 10-channel systems in order to form a 30-channel multiframe, and vice versa, increasing the complexity and cost of interfaces.